1. Field of the Invention
This invention relates to CDMA communication systems, and more particularly to methods for controlling forward link power during an intergenerational soft handoff in CDMA communication systems.
2. Description of Related Art
Wireless communication systems facilitate two-way communication between a plurality of subscriber mobile radio stations or “mobile stations” and a fixed network infrastructure. Typically, the plurality of mobile stations communicate with the fixed network infrastructure via a plurality of fixed base stations. Exemplary systems include such mobile cellular telephone systems as Code Division Multiple Access (CDMA) systems, Time Division Multiple Access (TDMA) systems, and Frequency Division Multiple Access (FDMA) systems. The objective of these digital wireless communication systems is to provide communication channels on demand between the mobile stations and the base stations in order to connect the mobile station users with the fixed network infrastructure (usually a wired-line system).
Exemplary CDMA Communication System
Mobile stations typically communicate with base stations using a duplexing scheme that allows for the exchange of information in both directions of connection. In most existing communication systems, transmissions from a base station to a mobile station are referred to as “forward link” transmissions. Transmissions from a mobile station to a base station are referred to as “reverse link” transmissions. These CDMA systems are well known in the art. For example, one such system is described in U.S. Pat. No. 4,901,307, issued on Feb. 13, 1990 to Gilhousen et al., which is hereby incorporated by reference for its teachings of CDMA communication systems.
Basic radio system parameters and call processing procedures for exemplary prior art CDMA systems are described in the TIA specification, entitled “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” TIA/EIA/IS-95-A, published in May 1995 by the Telecommunications Industry Association, and referred to hereafter as “IS-95A”. The update and revision to IS-95A and J-STD-008 (PCS specification analogous to IS-95A) is TIA/EIA/IS-95-B, which was published in March 1999 by the Telecommunications Industry Association (TIA), and is referred to hereafter as “IS-95B”. The IS-95A and IS-95B specifications are jointly known as specifying the second generation or “2G” CDMA system. More recently, a third generation, or “3G” CDMA system, is described in the TIA specification, and is entitled “cdma2000 Series”, TIA/EIA/IS-2000-A. The TIA/EIA/IS-2000-A specification was published in March 2000 by the TIA, and is referred to hereafter as “IS-2000”. The IS-95A, IS-95B and IS-2000 specifications are hereby incorporated by reference for their teachings on CDMA communication systems.
FIG. 1 depicts a simplified depiction of the coverage area in CDMA communication systems having a service area comprising seven base stations controlled by a single MTSO 20. As shown in FIG. 1, a typical CDMA communication system comprises at least one mobile station and a plurality of fixed base stations geographically distributed over the system's service area and controlled by a mobile telecommunications switching office (MTSO) 20. The service area is defined as the geographical area within which a mobile station can remain and still communicate (i.e., maintain a valid radio link) with the CDMA communication system. Each base station provides communication services to a fixed area within the service area. The service area is known as the base station “coverage area”. When a mobile station is within a base station's coverage area, the base station is able to provide communication services to the mobile station. A base station that provides service to the mobile is also known as a “serving” base station. The MTSO 20 coordinates all of the switching functions between base stations, mobile stations, and other communications systems (e.g., a Public Service Telephone Network or satellite communication system).
Communication between a base station and a mobile station is established by a negotiation process that is initiated upon call origination. The serving base station begins the negotiation process by assigning a selected one of its available forward traffic channels to the mobile station and thus establishes a forward link with the mobile station. The mobile station then establishes a reverse link with the serving base station. Once communication is established between the serving base station and the mobile station, pilot channels emitted by each base station are used by the mobile station to determine which base station coverage area the mobile station belongs to and the quality of the radio link with the base station. Specifically, each base station transmits an unmodulated pilot channel on a predetermined frequency that aids the mobile stations in detecting signals and measuring signal strengths of nearby base stations.
Power Control in CDMA Communication Systems
Power control is an important operational consideration in CDMA communication systems. A single wideband channel is reused in every cell in a typical CDMA system. For example, in the system of FIG. 1, CDMA cell coverage areas 1–7 operate on a single wideband channel. As is well known in the CDMA communication art, the capacity of CDMA systems capacity is largely limited by interference caused by wideband channel reuse (or frequency reuse). Specifically, system capacity is limited by the interference caused by mobile users transmitting in the same cell and by the interference caused by interferers in other cells. CDMA systems attempt to limit the interference caused by frequency reuse by controlling transmitter output power such that all transmitted signals arrive at a CDMA receiver with equal average power. Specifically, CDMA communication systems dynamically control the power of the mobile station and base station transmitters. When a mobile station changes position relative to its serving base station, its transmitter output power must be adjusted to maintain a transmitter output power equal to the average power level of all transmitted signals. Therefore, power is dynamically controlled. An overview of the dynamic power control used in CDMA communication systems is now described.
FIG. 2 shows a simplified block diagram of an exemplary CDMA communication system. As shown in FIG. 2, the exemplary CDMA communication system comprises a mobile station 10 and a mobile telecommunications switching office (MTSO) 20 controlling a base station 12. The mobile telecommunications switching office (MTSO) 20 controls the mobile station 10 and the base station 12. The MTSO 20 comprises a base station controller (BSC) 22 subsystem and a mobile station controller (MSC) 24 subsystem. The BSC 22 controls all of the base stations that are associated with the MTSO 20. Similarly, the MSC 24 controls all of the mobile stations that are associated with the MTSO 20. The mobile station 10 communicates with the base station 12 (i.e., its serving base station) on a forward traffic channel (FTC) 30 and a reverse traffic channel (RTC) 32. The base station 12 transmits the FTC 30 to the mobile station 10. The mobile station 10 transmits the RTC 32 to the base station 12.
CDMA communication systems control power by sending power control commands between the mobile stations and their associated base stations. Referring to FIG. 2, the base station 12 controls the transmitter power output of the mobile station 10 using the following technique. The base station 12 measures the signal strength of the RTC 32, which is proportionally related to the output power of the mobile station 10. If the base station 12 determines that the signal strength of the RTC 32 requires an adjustment, the mobile station transmitter output power is adjusted according to power control commands transmitted to the mobile station 10. As shown in FIG. 2, the base station 12 transmits power control commands to the mobile station 10 over a reverse power control channel (RPCC) 34. The RPCC 34 is punctured into over the FTC 30. Power output control of the base station 12 is achieved in a similar technique described below.
The base station 12 controls its transmitter power output using the following technique. The mobile station 10 measures the signal strength of the FTC 30, which is proportionally related to the output power of the base station 12. The mobile station 10 transmits the signal strength measurements to the base station 12. If the base station 12 or the mobile station 10 determines that the signal strength of the FTC 30 requires adjustment, the base station transmitter output power is adjusted according to power control commands that are transmitted to the base station 12. As shown in FIG. 2, the mobile station 10 transmits power control commands to the base station 12 using a forward power control channel (FPCC) 36. The FPCC 36 is punctured into the reverse pilot channel.
CDMA Handoffs
CDMA handoffs occur when a mobile station moves from the coverage area of its active base station to the coverage area of a new base station. In typical CDMA systems, a mobile station maintains a list of available base stations for providing communication services to the mobile station. Normally, the mobile station communicates with a base station having the strongest signal. The mobile station receives the pilot signals and determines which pilot signals are the strongest. A “searcher” unit in the mobile station commonly performs the signal detection and strength measurement functions.
The results of the searcher function are reported to the current (i.e., the active) base station. The base station then instructs the mobile station to update a list of available base stations that are maintained by the mobile station. The list is sub-divided into three operative sets—an “active set”, a “candidate set”, and a “neighbor set”. The active set contains a list of the base stations with which the mobile station is currently communicating (typically 1–4 base stations). The candidate set contains a list of base stations that may move into the active set. The neighbor set contains a list of base stations that are being monitored, albeit on a less frequent basis.
As the mobile station moves and its currently active base station signal weakens, the mobile station must access a new base station. Based upon the results of the searcher function, and the instructions received from the base station, the mobile station updates its sets, and communicates with a different base station. In order for communication transmissions to appear seamless to the user of the mobile station, the communication link must be “handed off” to the next base station. A handoff occurs when a mobile station moves across a “boundary line” from a first serving base station coverage area to a second base station coverage area. The communication system “hands over” or transfers service from the first serving base station to the second base station, also known as the “target” base station. A handoff also occurs when a single base station utilizes multiple frequency channels and switches communication between frequency channels. Each pilot channel is identified by a pseudo-random noise (PN) sequence offset and a frequency assignment. Thus, each pilot channel is uniquely identified with a base station that transmits the pilot channel. Pilot channels aid mobile stations in performing handoffs.
Referring again to FIG. 1, each base station services a separate coverage area, represented by a hexagon, and communicates with a specific frequency, a frequency one (F1) or a frequency two (F2), on a single wideband channel. Examples of wideband channels used by CDMA systems include the well-known Cellular (800 MHz) and PCS (1900 MHz) bands. Other wideband channels can be used without departing from the spirit or scope of the present invention. In the exemplary CDMA system of FIG. 1, a first base station 12, located in the middle of a Service Coverage Area One, communicates on a first frequency F1 A mobile station 10 located in Coverage Area One and therefore is serviced by the first base station 12. When the mobile station 10 moves from Coverage Area One to a Coverage Area Two, it performs a handoff procedure from the first base station 12 (the serving base station) to a second base station 14 (the target base station). Thus, the mobile station 10′ is serviced by the second base station 14.
There are two basic types of handoffs in CDMA systems: “hard handoffs” (HHO) and “soft handoffs” (SHO). A “soft handoff” or “Make-Before-Break” handoff is a handoff procedure in which the mobile station commences communication with a target base station without interrupting communication with the serving base station. Because mobile stations typically contain only one transmitter, soft handoffs can only be used between base stations with CDMA Channels having identical frequency assignments. Referring again to FIG. 1, a soft handoff procedure is performed when the mobile station 10 travels from a first Coverage Area One to a third Coverage Area Three because the base station 12 and a third base station 16 have identical frequency assignments, F1.
A “hard handoff” is defined as a handoff in which a mobile station commences communication with a target base station after a momentary interruption in communication with a serving base station. Hard handoffs are also referred to as “Break-Before-Make” handoffs. A hard handoff is used when the serving base stations and the target base stations have differing CDMA channel frequency assignments. As shown in FIG. 1, the first base station 12 is assigned a first frequency F1 and the second base station 14 is assigned a second frequency F2. A hard handoff is performed when the mobile station 10 travels from the Coverage Area One to the Coverage Area Two because the first base station 12 and the second base station 14 operate on different frequencies, F1 and F2, as shown in FIG. 1.
A hard handoff can also occur when a single base station utilizes multiple frequency channels and switches communication between frequency channels. For example, a single base station hard handoff can occur between sectors associated with a single base station. The present invention is concerned with the multiple base station scenario, and thus, the single base station scenario is not described in detail herein. However, those skilled in the art shall recognize that the present invention can be utilized equally as well in a single base station scenario.
During a hard handoff, the radio link is momentarily interrupted because a typical mobile station contains only one transmitter and therefore can only demodulate one frequency at a time. Thus, switching from the CDMA channels of the serving base station frequency to the CDMA channels of the target base station frequency produces a momentary interruption in the continuity of the radio link with the CDMA communication system. As described in more detail below, this momentary interruption can result in improper initial power transmissions occurring between a mobile station and its new serving base station. These power transmissions are initially improper because a mobile station and a new serving base station do not have information regarding the others transmitter output power. Improper power transmissions adversely affect a CDMA communication system quality of service and capacity. Intergenerational handoffs are now described.
Intergenerational CDMA Handoffs
Handoffs performed between different generation CDMA systems (e.g., within an intergenerational CDMA system having both 2G CDMA systems and 3G CDMA systems) are known as “intergenerational handoffs” (IGHO). An exemplary intergenerational CDMA system and IHO is described in more detail below with reference to FIG. 3. An IHO can be a soft handoff or a hard handoff. 3G CDMA systems have been designed to provide backward compatibility with 2G CDMA systems at the signaling and call processing level. However, 2G and 3G CDMA systems are not naturally compatible at the physical layer because these systems employ different modulation schemes and spreading rates. Thus, due to the incompatibility between 2G and 3G CDMA systems, problems can occur when performing IHOs. For example, a “complete” Intergenerational Soft Handoff (ISHO) (i.e., a soft handoff on both the forward reverse links) is presently not feasible.
Intergenerational CDMA systems (e.g., CDMA systems comprising both 2G and 3G CDMA systems) can perform forward link ISHOs because mobile stations typically comprise “rake” receivers that are capable of concurrently demodulating multiple signals. Thus, a typical mobile station can simultaneously demodulate a signal from a 2G serving base station and a signal from a 3G target base station. Rake receivers and simultaneous demodulation techniques are well known in the CDMA art and thus are not described in detail herein. However, reverse link ISHOs cannot presently occur because a typical mobile station has only one transmitter. Due to the intergenerational incompatibility described above, the typical mobile station can only communicate simultaneously on the reverse link with base stations of the same generation. Therefore, at the service boundaries between the 2G and 3G systems, a reverse link intergenerational hard handoff (IHHO) has been proposed.
In this type of hard handoff, the connection with a currently active base station (e.g., 2G) is terminated before a new service with a new base station (e.g., 3G) is established. As described in more detail below, this momentary interruption during a reverse link IHHO can result in an improper initial power transmission from the mobile station to its new serving base station. Power control techniques used when performing intergenerational soft handoffs is now described.
Power Control During Intergenerational Soft Handoffs (ISHO)
One of the necessary capabilities of CDMA communication systems is dynamic power control of mobile and base station transmitters. During an ISHO, the CDMA system performs a forward link ISHO and a reverse link IHHO. As described in more detail below with reference to FIG. 3, a reverse link IHHO can result in the occurrence of an improper initial power transmission from the mobile station to its new serving base station, and vice versa.
FIG. 3 shows a simplified block diagram of an exemplary intergenerational CDMA communication system. FIG. 3 is substantially similar to FIG. 2 and thus identical items are not described. As shown in FIG. 3, the exemplary CDMA communication system comprises the mobile station 10 and the mobile telecommunications switching office (MTSO) 20 the MTSO 20 controls the base station 12 (a 2G CDMA system) and a base station 14 (a 3G CDMA system).
The 2G base station 12 communicates with the mobile station 10 using the forward traffic channel (FTC) 30 and the reverse traffic channel (RTC) 32. As shown in FIG. 3, the 2G base station 12 sends power control commands to the mobile station 10 using the reverse power control channel (RPCC) 34. The mobile station 10 transmits power control commands to the 2G base station 12 using the forward power control channel (FPCC) 36. The 3G base station 14 communicates with the mobile station 10 using an FTC 40 and an RTC 42. As shown in FIG. 3, the 3G base station 14 sends power control commands to the mobile station 10 over a reverse power control channel (RPCC) 44. The mobile station 10 transmits power control commands to the 3G base station 14 over a forward power control channel (FPCC) 46.
An ISHO is described with reference to the exemplary intergenerational CDMA system of FIG. 3. Initially, the mobile station 10 communicates with its serving base station 12 over the FTC 30 and the RTC 32. As described above with reference to FIG. 2, the exemplary intergenerational CDMA communication system controls the transmitted power of both the base station and mobile station by transmitting power control commands between the mobile station 10 and the serving 2G base station 12 over the FPCC 36 and the RPCC 34.
The CDMA system initiates an ISHO when the mobile station 10 approaches the coverage area of the 3G base station 14. Thus, a forward link ISHO is performed by establishing communication from the 3G base station 14 to the mobile station 10 via the FTC 40. The forward link is in “soft handoff” because the mobile station 10 simultaneously communicates with the 2G base station 12 and the 3G base station 14. Due to the limitation that typical mobile stations have only one transmitter, the RTC 42 cannot be established while the mobile station 10 is still communicating with the 2G serving base station 12. Thus, the FPCC 46 cannot be established over the RTC 42 until a reverse link IHHO is performed. Disadvantageously, the reverse link IHHO disrupts power control processing, which can result in a severe decrease in the Quality of Service “QoS” of all mobile stations and the overall capacity of the CDMA system. During a reverse link IHHO, the target base station may request a power increase in the RTC of the mobile station because the target base station erroneously measures signal energy from a “nonexistent” RTC. The RTC of the target base station is nonexistent because the mobile station is communicating on the RTC of the serving base station. In response to the target base station request for a power increase in the RTC, the mobile station may increase its RTC power to the serving base station. Also, the target base station may once again measure the nonexistent RTC of the mobile station and will not detect the requested increase in signal energy. In response, the target base station may increase its FTC transmit power because it may assume that the mobile station has an impaired channel condition (i.e., as caused by shadow fading). These improper power conditions may result in the target base station transmitting too much power on the newly formed FTC. The increased power condition may change the power balance at the input of the new serving base station receiver. The new serving base station will, in turn, increase the transmit power of all associated mobile stations. The erroneous power conditions significantly decrease the QoS and capacity of the CDMA system. Thus, it is desirable to provide a method and apparatus for controlling power between two different generations of CDMA systems during intergenerational handoffs. The present invention provides such a method and apparatus for controlling forward link power during intergenerational soft handoffs in a CDMA communication systems.